Method and apparatus for cleaving anisotropic pyrolytically deposited materials on curved surfaces

ABSTRACT

A METHOD OF SEPARATION OF ARTICLES DEPOSITED BY GAS PYROLYSIS FROM TERMINATION PORTIONS FORMED DURING MANUFACTURE. THE SEPARATION OF THE CENTER PORTION OF THE DEPOSITED ARTICLE FROM THE TERMINATION PORTIONS NORMALLY FORMED DURING THE DEPOSITION PROCEDURE IS FACILIATED BY FORMING A RADIALLY INWARDLY EXTENDING ROUNDED ANNULAR SHOULDER ADJACENT TO AND ON THE TERMNATION SIDE OF AN ANNULAR FLANGE USED TO CREATE A FAULT IN THE DEPOSITED ARTICLE. THE COMBINATION OF THE ROUNDED SHOULDER AND THE FLANGE PRODUCES A FAULT IN THE PYROLYTICALLY DEPOSITED MATERIAL AND STRESES ON EITHER SIDE OF THE FAULT THAT ARE OPPOSITE IN DIRECTION WHICH RESULTS IN A CLEAN SEPARATION OF THE TERMINATION PORTION FROM THE CENTER PORTION OF THE DEPOSITED PPYROLYTIC ARTICLE.

3,657,404 OLYTICALL B. L. ETTINGER Aprll 18, 1972 METHOD AND APPARATUS FOR CLEAVING ANISOTROPIG PYR DEPOSITED MATERIALS 0N CURVED SURFACES Original Filed Oct. 3, 1966 F/l/l l/ll/l/Il/ll/l /////A 1 N VENTOR L fffz'zfyer mce BY Mar M United States Patent 3,657,404 METHOD AND APPARATUS FOR CLEAVING ANTSOTROPIC PYROLYTICALLY DEPOSITED MATERIALS 0N CURVED SURFACES Bruce L. Ettinger, Detroit, Mich., assignor to General Electric Company Continuation of application Ser. No. 583,814, Oct. 3, 1966. This application May 28, 1969, Ser. No. 833,846 Int. Cl. C04b 35/52, 35/54, 35/58 US. Cl. 264-81 13 Claims ABSTRACT OF THE DISCLOSURE A method of separation of articles deposited by gas pyrolysis from termination portions formed during manufacture. The separation of the center portion of the deposited article from the termination portions normally formed during the deposition procedure is facilitated by forming a radially inwardly extending rounded annular shoulder adjacent to and on the termination side of an annular flange used to create a fault in the deposited article. The combination of the rounded shoulder and the flange produces a fault in the pyrolytically deposited material and stresses on either side of the fault that are opposite in direction which results in a clean separation of the termination portion from the center portion of the deposited pyrolytic article.

This application is a continuation of application Ser. No. 583,814 filed on Oct. 3, 1966 and now abandoned.

The subject matter of the present invention is a method and apparatus for forming articles by gas pyrolysis, the key feature of which is an improved method and means for assuring good separation of the formed articles from termination portions inherently formed during manufacture, with minimum hazard of damage to the article during such separation. The invention has particular utility in the manufacture of pyrolytic graphite articles and will be described in detail specifically with reference thereto; however, it will be understood that the invention, in its broader aspects, is applicable to the manufacture of other types of pyrolytic articles, examples of which will be given hereinafter.

Pyrolytic graphite is manufactured by pyrolysis, or thermal decomposition, of carbonaceous gas. Any of a wide variety of carbonaceous gases may be used, though in practice methane either alone or in combination with hydrogen is preferred. To make a tube of pyrolytic graphite, for example, the carbonaceous gas is passed through a tubular shaped mandrel, preferably of ordinary Acheson graphite having a controlled relatively smooth inner surface finish, the mandrel being heated to a sufficiently high temperature to cause pyrolysis of the carbonaceous gas with resultant deposition of the pyrolytic graphite on the interior wall of the tubular mandrel. The graphite deposits in laminae and the process is continued until the desired thickness is accomplished. Upon cooling, the mandrel cracks due to the difference in coefficients of thermal expansion and can thereby be removed from the deposited article.

Pyrolytic graphite is anisotropic and, by reason of this fact and its extremely high temperature resistance and also its nuclear properties has a broad field of utility such, for example, as for lamp filaments, furnace linings, nuclear reactor moderator rocket nozzles and re-entry heat shields. The latter two uses have become particularly important in recent years and generally require that the pyrolytic graphite article be of tubular or circular cross-section.

The properties of the pyrolytic graphite formed are determined, at least in part, by the temperature, pressure SfiSMM Patented Apr. 18, 1972 and composition of the carbonaceous gas and also by the concentration and therefore the rate of flow of the carbonaceous gas as it contacts the heated mandrel and un dergoes pyrolysis. As a practical matter, it is impossible, or at least would be prohibitively expensive, to so control all of these variables throughout the length of the deposit as to attain a pyrolytic graphite article of absolutely uniform properties from one end to the other thereof. For example, temperature and concentration gradients will inherently be present along the tubular mandrel and the terminations thereof when, as in conventional techniques, the carbonaceous gas is flowed into one end of the mandrel with the pyrolysis by-products exiting from the other end. Hence, it is the common practice to use a mandrel assembly consisting of a center tubular portion which portion is commonly referred to as the mandrel and with appendages at the ends thereof so as to form the pyrolytic article with integral termination portions which can be later separated from the center portion of the deposit, when the appendages are removed from the mandrel, the center portion of the deposit constituting the desired article of manufacture. Since excellent control of temperature, pressure, and gas concentration and flow rate can be accomplished in the center portion of the mandrel assembly; i.e. in the mandrel itself, the resultant center portion of the pyrolytic graphite deposit can be of the desired uniform properties from one end to the other, the non-uniform termination portions resulting from the temperature, concentration and other gradients at the appendages of the mandrel being later removed and discarded. The difficulty, however, is that the non-uniform properties or poor quality of the termination portions often result in the occurrence of delaminations or cracks in these portions and such cracks or delaminations will frequently propagate to the desired center portion of the article prior to or during separation of the center portion from the termination portions. This results in a high scrap loss which is an important factor in the relatively high cost of producing good quality pyrolytic graphite rocket nozzles, re-entry heat shields and the like.

To alleviate the problem it has been proposed to construct the mandrel assembly with a pair of inwardly extending annular flanges, one adjacent each end of the mandrel at the juncture between the center portion and the appendage portion. Since the pyrolytic graphite deposits in successive layers coplanar with the portion of the mandrel surface on which it is deposited and since each flange provides two adjacent surfaces at an angle of say about to each other, the resultant deposit at the location of each flange has a structural fault extending through the laminations which circumscribes the tubular pyrolytic graphite article and has a frusto-conical section which bisects the angle between the radially inwardly extending flange and the adjacent cylindrical wall portion of the mandrel. These structural faults, often referred to as cleavage planes, have a low shear strength and are intended to provent the propagation of cracks or delaminations from the termination portions to the center portion of the deposited pyrolytic graphite article. However, this technique has proved only partially successful since it often occurs that a crack or delamination in a termination portion will propagate through the intentionally created fault and into the center portion of the article. Hence, at the present state of the art there continues to be a high scrap loss in the manufacture of pyrolytic graphite articles and there is need for an improved method and means for providing a clean break between the desired center portion and the termination portions of the deposited article without propagation of cracks or other failures from the termination portions to the center portion. The present invention fills this need.

Briefly, what I have discovered is that if the surface of the mandrel assembly adjacent to and on the termination side of the inwardly extending annular flange is formed with a radially inwardly extending rounded annular shoulder, the inner surface of such shoulder thereby having a concave-convex compound curvature, the combination of the rounded shoulder and the flange enables a clean separation of the termination portion from the center portion of the deposited pyrolytic graphite article at the fault resulting from the inwardly extending annular flange. In the most preferred embodiment of the invention a pair of radially inwardly extending annular flanges is used adjacent each end of the mandrel assembly, one flange on one side of the compound curved shoulder and the other flange on the other side of the compound curved shoulder thereby providing faults in the deposited pyrolytic graphite article on each side of the shoulder. As will be discussed in greater detail hereinafter, I have found that the radially inwardly extending rounded shoulder results in a pyrolytic graphite article wherein there is a substantial difference in stress between those portions of the article to either side of and immediately adjacent the fault. More specifically, that portion of the article deposited over the inwardly extending compound curved shoulder, by way of residual stresses, is stressed in tension on its outside surface whereas the center portion of the deposited article which is to the other side of the fault and which is of simple curvature, is again by way of residual stresses, stressed in compression on its outside surface. Hence, this provides a favorable difference in stress between the deposited portions to either side of the fault, such portions being stressed in opposite directions. With such differences in stress to either side of the fault, it frequently occurs that upon completion of the deposition and removal of the mandrel, the termination portions separate cleanly from the center portion of the article without any need to even score or cut the article to initiate such separation. When such separation does not immediately occur, it is a simple matter to cause the clean separation by initiating a crack or cut in the termination portion, such cut or crack propagating to the fault and then causing separation of the termination portion from the center portion at the fault.

Other objects, features and advantages of the invention will appear more clearly from the following detailed description of preferred embodiments thereof made with reference to the drawings in which:

FIG. 1 is a side view in section of a furnace incorporating a mandrel embodying the invention;

FIG. 2 is a side view in section of a portion of the apparatus shown in FIG. 1 but after deposition of the pyrolytic graphite article;

FIG. 3 is an enlarged sectional view of a portion of the mandrel and deposited article shown in FIG. 2; and

FIG. 4 is a sectional view of a portion of another embodiment of the invention.

Referring now to FIG. 1, the apparatus shown comprises a generally cylindrical casing having enclosure plate 12 which is removably secured as by bolts or a suitable hinge and latch. A viewing window 13 enables inspection of the deposition operation within the casing and also viewing with an optical pyrometer. A body of insulating material 14, such as carbon black, defines an inner cylindrical chamber, the walls of which are formed by a graphite cylinder 18 and top and bottom graphite plates 20 and 22 respectively. An induction heating coil 24 surrounds the insulating material 14, the graphite cylinder 18 functioning as the susceptor whereby intense heat is generated within the cylinder 18 by reason of the passage of current through the induction coil 24.

Extending through the heating chamber defined by the cylinder 18 and its end plates, is a mandrel assembly 25 constructed in accordance with the invention and to be described in detail hereinafter. An opening in plate 22 accommodates an inlet tube 26 for the flow of carbonaceous gas into and through the mandrel, the upper end of the mandrel assembly being open to the interior of the casing 10 whereby the non-deposited products of the pyrolysis of the carbonaceous gas can exit through the outlet tube 28. Hence, in operation, the carbonaceous gas such as methane or a mixture of methane and hydrogen is admitted through tube 26 to the interior of the mandrel assembly which is intensely heated by the heat generated by the cylinder 18. Pyrolysis of the carbonaceous gas thereby occurs with resultant deposition of pyrolytic graphite on all of the interior walls of the mandrel assembly, the hydrogen and other gaseous pyrolysis products being withdrawn from the chamber through the outlet tube 28. As is well known in the art, temperatures on the order of 1200 to 2500 C. can be used to cause the pyrolysis reaction.

In accordance with the present invention, the mandrel assembly comprises a tubular mandrel 30 with top and bottom appendages which, in the embodiment shown, are of substantially the same structure. The top appendage comprises spaced radially inwardly extending annular flanges 32 and 34 with a curved radially inwardly extending annular shoulder 36 therebetween, and a tube portion 38 extending upwardly from the flange 34. The entire mandrel assembly including its appendages can, if desired, be of one integral piece with the interior shape of the appendages as shown, and the term mandrel assembly is intended to comprehend a unitary construction as well as a multiple part construction. However, for convenient low cost manufacture of the mandrel assembly, it is preferable that it can be made in separate parts as shown. For simplification of assembly of the mandrel pieces, each of the flanges 32 and 34 has an outer cylindrical portion, as shown at 40, (see FIG. 2) which has an internal diameter the same as the external diameter of the parts 30, 36 and 38 to which the flange is assembled. This assures good alignment of the assembled parts. All of the mandrel assembly parts are formed of graphite, preferably ordinary electro-graphite with the interior surfaces machined to desired smoothness. The lower mandrel appendage comprising flange parts 42 and 44, rounded shoulder part 46 and tube part 48, is of substantially the same structure as described above except that in the particular embodiment shown the tube part 48 is shorter than the tube 38 of the upper appendage and rests against the inner surface of plate 22 as distinguished from the upper appendage wherein the tube 38 extends through the plate 20 to the exterior of the insulation body 14.

Each of the flanges 32, 34, 42 and 44, extends radially inwardly at an angle to the abutting cylindrical wall portions, this angle at its apex being in the particular embodiment shown in FIG. 1 through, as will be mentioned hereinafter, such angle can be anywhere from 30 to The length or, in other words, the radially inward extent of the flanges should preferably be at least equal to the wall thickness of the pyrolytic graphite article desired to be deposited.

Each of the radially inwardly extending rounded shoulders 36 and 46 provides an inner surface which is of compound curvature, the curvature in a plane transverse to the mandrel being concave and the curvature in a plane through the longitudinal axis of the mandrel being convex and with the radius of convex curvature being smaller than the radius of concave curvature.

Operation of the apparatus is as described above and after deposition is complete the pyrolytic graphite article with its termination portions will be deposited on the interior surfaces of the mandrel assembly as shown in FIG. 2. The center portion 49 deposited on the mandrel 30 constitutes the desired pyrolytic graphite article and the portions 50 and 52, deposited on the mandrel appendages, constitute the termination portions of the deposit which are desired to be separated from the center portion 49.

The laminar structure of the deposit on each of the mandrel appendages can best be seen in FIG. 3 which shows, in enlarged scale, a portion of the top mandrel appendage and the general laminar structure of the pyrolytic graphite deposit thereon. It will be noted that to each side of each of the flanges 32 and 34 a structural fault extending through the laminations and of general frustoconical shape is formed at an angle which generally bisects the angle between the flange and its abutting cylindrical wall portion, such faults being designated by the reference numerals 54, 56, 58 and 60. Each of these faults results from the growth of the laminae coplanar to the surface on which they are deposited, fault 54, for example, defining the line of juncture between the laminations deposited on the wall 38 and those deposited on the upper surface of the flange 34.

Those portions of the pyrolytic graphite article, and its extensions, deposited on the cylindrical or simple curved portions of the mandrel assembly are stressed in compression on their outside surfaces. This is evidenced by H the fact that if a wall of the cylindrical portion 49 is cut in the direction of the longitudinal axis of the cylinder, the resulting adjacent Wall edge portions will tend to curl inwardly to assume a shorter radius of curvature. The same occurs if either of the cylindrical portions deposited on mandrel parts 38 or 48 is split or cut axially through a wall thereof. However, the two portions of the article extensions deposited on mandrel parts 36 and 46 are stressed in tension on their outside surfaces as evidenced by the fact that if these portions are cut in an axial direction through the wall thereof, the resulting abutting wall edge portions tend to spread apart to assume a larger radius of curvature. In effect, then, there is a substantial difierence in the stress condition of the deposit to either side of each of the flanges, 32 and 34 (see FIG. 3) and the associated faults, the portions of the deposit on mandrel assembly parts 30 and 38 being stressed in compression and the portion of the deposit on mandrel assembly part 36 being stressed in tension.

After the pyrolytic graphite deposition is completed, upon cooling, the mandrel assembly and the deposited article are removed from! the deposition chamber and the mandrel assembly is removed from the deposited article. It generally occurs that upon cooling the mandrel assembly will crack and fall away from the deposited article though if this does not occur the mandrel parts can be easily cut away from the pyrolytic graphite article. Further, by use of the mandrel assembly as shown and described, it often occurs that the undesired termination portions of the deposited article voluntarily break away cleanly from the desired center portion of the deposit at the inner faults formed at the flanges 32 and 42. If such separation does not immediately occur, it can be caused to occur simply by cutting, cracking or jarring the undesired termination portions, the cuts or cracks thereby initiated immediately propagating to the faults and cansing a clean break between the desired center portion of the deposit and the termination portions at the inner faults formed at the flanges 32 and 42. The inner fault formed at flange 32 is shown at 60 in FIG. 3. It will be understood, of course, that the low quality termination portions of the deposit which are separated from the desired center portion 49 will often have cracked or delaminated into a number of pieces at the time the separation is complete.

Whereas the embodiment described above with reference to FIGS. 1 through 3 is preferred, any of various other embodiments of the invention can be used if desired. For example, flanges 34 and 44. can be eliminated. Also, any or all of the flanges used can, if desired, extend at angles other than 90 to the cylindrical surface of the mandrel assembly, any angle of from 30 to 150 being satisfactory. The shape of any of the flanges can be such that the top and bottom angles formed thereby with the cylindrical mandrel wall are the same, or are different. For example the upper angle might be and the lower angle 60, this by a flange which has parallel upper and lower surfaces and which extends slightly upwardly, or downwardly; or the upper and lower angles can both be, say, 75 by using a flange having a wedge shaped cross section.

Still another embodiment of the invention is shown in FIG. 4. In this embodiment each of the upper and lower appendages of the mandrel assembly consist of an inwardly extending rounded shoulder 62 of concave-convex compound curvature, such shoulder having upper and lower surfaces 64 and 66 which form apex angles with cylindrical mandrel portions 68 and 70, respectively, of from 30 to- In the particular structure shown in FIG. 4, the upper and lower angles are both about 110. With such structure, faults 72 and 74 form to each side of the shoulder in the deposited article, the portions of the article deposited within mandrel portions 68 and 70 being stressed in compression on their outside surfaces and the portion of the article deposited on the compound curved shoulder 62 being stressed in tension on its outside surface. Hence, this provides the requisite fault with the portions deposited to either side thereof being under greatly different stress.

As alluded to above, whereas the invention has its greatest utility and has been described specifically with reference to the manufacture of pyrolytic graphite articles, it will also find utility in the manufacture of pyrolytic articles of other anisotropic materials. Such additional materials can include the refractory metals of Groups IV, V and VI of the Periodic Table and their carbides, borides, nitrides and silicides such as those of hafnium, molybdenum, niobium, silicon, tantalum, tung sten, titanium and zirconium. As is well known in the art, the gas composition used for the pyrolysis reaction to form such pyrolytic articles will vary depending upon the precise material desired. For example, the aforesaid metals may be pyrolytically deposited by using as starting materials the halogenated derivatives of the metals whereas to form the nitrides or carbides, for example, the starting materials will consist of a mixture of the metal derivative plus ammonia or carbonaceous gas, respectively.

In its broader scope, therefore, the invention will find utility for the manufacture of any pyrolytic article wherein there exists the problem of obtaining good separation of the desired center portion of the deposited article from its termination portions. Hence, it will be understood that while the invention has been described specifically with reference to preferred embodiments thereof, various changes and modifications maybe made all within the full and intended scope of the claims which follow:

I claim:

1. A method for manufacturing an article of anisotropic pyrolytic material selected from the group consisting essentially of graphite and refractory metals of Groups IV, V and VI of the Periodic Table and their carbides, borides, nitrides and silicides, comprising the steps of depositing on a surface the material to a shape which provides a fault therethrough, said fault being created by a radial flange extending inwardly a distance at least equal to the thickness of the deposit, and which provides a radially inwardly extending rounded shoulder of concave-convex compound curvature adjacent one side of said fault which is stressed in tension on the outside surface thereof and a curved portion on the other side of said fault which is stressed in compression on the outside surface thereof adjoining said surface of said first-mentioned portion wherein in the concave-convex compound curvature the radius of the convex curvature is smaller than the radius of concave curvature, said opposite stresses causing cleavage at the fault at least upon cracking of one of said portions in a direction generally transverse to said fault,

7 separating said curved portion on one side of said fault from said curved portion on the other side of said fault and from said deposition surface.

2 A method as set forth in claim 1 wherein said material is deposited to a shape which provides a cylindrical portion on one side of said fault and a portion having a surface of concave-convex compound curvature on the other side of said fault.

3. A method as set forth in claim 1 wherein said fault is of generally frustoconical shape.

4. A method as set forth in claim 1 wherein said deposition is by pyrolysis of a carbonaceous gas to form a pyrolytic article of anisotropic graphite.

5. A method as set forth in claim 1 wherein said material is deposited to a shape having a cylindrical center portion and a radially inwardly extending annular flange between the shoulder and the center portion, there being a pair of generally frustoconical shaped faults formed by each of said flanges.

6. A method as set forth in claim 5 wherein said material is deposited to a shape having at each end thereof of a second radially inwardly extending annular flange, said second flange being on the other side of the shoulder and providing two additional frustoconical shaped faults.

7. In apparatus for forming a curved article of anisotropic pyrolytic material comprising a casing, pyrolytic material inlet means connected to said casing, a curved deposition surface within said casing, and heating means surrounding said deposition surface,

the improvement in which the curved deposition surface has a radially inwardly extending shoulder with an inner surface of concave-convex compound curvature wherein the radius of the convex curvative is smaller than the radius of the concave curvature, a portion adjacent said shoulder having an inner surface of simple concave curvature, and a generally radially inwardly extending flange between said surface of Simple curvature and said surface of compound curvature and which extends radially inwardly a distance at least equal to the thickness of the anisotropic article to be formed and which is at an angle 8 to said surface of simple curvature, said angle having an apex of from about 30 to 8. Apparatus as set forth in claim 7 wherein said radially inwardly extending surface is provided by a radially inwardly extending annular flange between said shoulder and said portion of simple curvature.

9. Apparatus as set forth in claim 7 wherein there is a portion of simple concave curvature on each side of said shoulder.

10. Apparatus as set forth in claim 9 wherein said radially inwardly extending surface is provided by a radially inwardly extending annular flange on one side of said shoulder and wherein there is a radially inwardly extending annular flange on the other side of said shoulder providing another radially inwardly extending surface which is at an angle at the apex thereof of from about 30 to 150 to one of said surfaces of simple curvature.

11. Apparatus as set forth in claim 7 wherein said radially inwardly extending surface is formed by a surface of said shoulder which adjoins said surface of simple curvature.

12. Apparatus as set forth in claim 7 wherein said shoulder portion and said portion of simple curvature constitute separate parts which are in abutting relationship.

13. Apparatus as set forth in claim 7 wherein said deposition surface is made of frangible graphite.

References Cited UNITED STATES PATENTS 3,265,519 8/1966 Diefendorf 117-8 3,324,212 6/1967 Paulley et a1. 264-63 3,410,746 11/1968 Turkat et al. 264-29 3,424,603 l/1969 Schwartz 264--29 3,457,042 7/1969 Ettinger 23209.l P

JULIUS FROME, Primary Examiner I. H. MILLER, Assistant Examiner US. Cl. X.R. 

